Heat transfer of rhyniophytic plant axes

Heat transfer is important for plants being sedentary organisms and exposed fully or partly to direct sunlight. It comprises three different mechanism s: (1) emission of IR (infra-red) radiation, (2) heat conduction and convec tion (sensible heat) and (3) evaporative cooling by transpiration (latent h eat). Transpiration has been shown to act as an efficient cooling device in the case of extant land plants. The earliest known land plants consist of a simple branching axis system without leaves or roots. This paper addresse s the question of how rhyniophytic plants dissipated heat as well as the si gnificance of evaporative cooling for these organisms. This is particularly interesting in light of the fact that rhyniophytic land plants show a low stomatal density compared to extant plants. Using formulae (representing ap proximative approaches) for forced convection (heat is 'carried away' by wi nd movements), the results suggest that if wind velocity is high enough for this heat transfer mechanism, then transpiration does not play a role in h eat dissipation. This is due to the fact that the slender habit of rhynioph ytic plant axes lead to high boundary layer conductance and that the transp iration rate is too low to significantly contribute to heat transfer. Durin g low wind velocities, the regime of mixed convection develops which leads to heat transfer both by forced convection and free convection (heat transf er by buoyancy plumes). Computer simulations were applied in order to study mixed convection for rhyniophytic plants due to the complexity of this hea t transfer regime. Slight air movements significantly decrease the plant te mperature due to the high boundary layer conductance. Although the transpir ation may be significant for heat transfer during low wind velocities if th e plant surface temperature is very high, convective heat transfer is expec ted to dominate heat dissipation. Further detailed investigations of the in teractions between a rhyniophytic plant stand and its micrometeorological e nvironment would be of great interest, because these plants differ from ext ant land plants in various properties which also affect microclimatic facto rs. Gaining new information about the ecophysiological behaviour of rhyniop hytic plants and their interactions with the microclimate created by these plants also concern other organisms associated with rhyniophytes, such as f ungi or arthropods. (C) 2001 Elsevier Science B.V. All rights reserved.